There are two basic types of filter elements used in pressure filters, one being a filter leaf as described, for example, in U.S. Pat. No. 3,212,643 and the other being a filter tube as described, for example, in the U.S. Pat. No. 3,233,739. In a typical filter leaf construction a pair of planar perforate members are mounted in parallel relationship on opposite sides of an underdrain disposed within an imperforate rigid support frame. In a typical filter tube construction a cylindrical perforate member surrounds an underdrain between top and bottom imperforate end caps. As the filtrate to be filtered flows into either of these types of filter elements through the perforate surfaces thereof, entrained solids are deposited over the perforations in the external surfaces, which surfaces may be covered by a perforate filter media, and gradually build up to form filter cakes. When the filter cakes have built up to a thickness where the efficiency of the filter is impaired, the filter is removed from the processing line, and the filter cakes are removed from the filter elements.
While the filter cakes can be removed in various ways, for most applications it is desirable to partially dewater the cakes before they are removed from the filter elements. If, however, the filter cakes have built up over the peripheral surfaces of the filter elements, it is difficult if not impossible to remove the water from the peripheral portions of the cakes. Where the moisture content of the cakes on the filter leaves is reduced by means of compression diaphragms of the type disclosed, for example, in U.S. Pat. Nos. 3,708,072 and 3,814,257, the edges of the diaphragms sometimes become embedded in the soft peripheral portions of the cakes which results in cake transfer to the diaphragms when the diaphragms are subsequently retracted away from the leaves. This problem can be particularly acute in the filtration of highly compressible materials such as organic sewage sludge where a peripheral cake may quickly build up after the diaphragm contacts the face of the cake. There are several other adverse effects which result from peripheral cake buildup, and they include increased moisture content of the removed cake, incomplete cake removal, failure of textile filter media, long down-times, and in some cases, improper operation of automatic control equipment.
Attempts at solving the problem of peripheral cake buildup have included such things as sealing off an upper peripheral portion of the filter media, and reducing the size of the filter element. Although neither method has resolved the problem, they both reduce the effective capacity of the filter. Sluicing systems for removing the soft peripheral portion of the cake while the cake is held in compression by a diaphragm have also been used. U.S. Pat. No. 4,064,045 provides a detailed description of one such system.